The real issue is that nutrition science is bad and everyone has a book. Maybe 1 in 20 studies don't have obvious confounding factors that cause us to have no real base facts, but we can find a lot of correlation. Unfortunately, this is kind of the nature of the beast, nutrition science isn't bad because nutrition scientists are bad, but rather because it's so complicated it's tough to have very clean data.
Demonizing fats in favor of sugar as a country has allowed one of the biggest and longest population studies of its kind, and it's pretty obvious from the health of the population that very high calorie, sugar rich diets are overall bad for you (notice the wide strokes we have to make here).
Everyone is different, and no rules apply uniformly to everyone, but general rules do apply to everyone. Fructose in a caloric surplus is more damaging than glucose, for instance, but different bodies are more or less effective at mitigating this. Most people have some inflammation response to gluten, but for the vast majority of people, it's minor. Most things are a sliding scale, on one end is no measurable result, on the other end is acute response.
NOTE: I'm not interested in arguing about what the research says here. I know the pro-gluten party will come out in force, and I've made an enemy with the bread baker's union. I'm not a researcher, I'm just a nutrition hobbyist. There are much smarter people doing much more comprehensive breakdowns and having much more meaningful debates than I can, so go listen to them. Read the research yourself with a skeptical eye. Take all dietary advice with a grain of salt, and check the opposite side of the research, nutrition and biology is WEIRD.
Also, we're still learning very basic things that have massive impacts. No one was talking about "gut health" unless you had a chronic disease roughly 10 years ago. Microbiome didn't have much place in the conversation about health and nutrition, no crossfit people knew about resistant starches, etc.